The present invention relates to knife assemblies for disk-type chippers, and in particular to an improved cassette knife assembly employing reversible blades.
Chippers of this type are well known, and one example is described in U.S. Pat. No. 4,784,337 to Nettles et al. The chipper there described is of the well-known Carthage-Norman design in which a vertical disk rotates upon a horizontal drive shaft. The disc is covered by a housing with a removable head, and a spout or feeder projects from the housing for guiding logs. The logs are supplied, generally end-on, to a series of radially disposed knives carried on the disk. The knives are clamped in place at cutting stations around the disk and chip slots or passages through the disk are situated at the cutting stations adjacent the knife blades. The wood chips cut from the logs move through the passage and are ejected from the machine via a casing discharge area. The spout or feeder has a stationary bedknife that cooperates with the rotary knives so that the logs are cut into uniform chips suitable to be supplied, e.g., to a digester of a wood pulping process.
The blades of the knife assemblies are the parts of the chipper most subject to wear. Ideally, the blades should be easy to replace and to adjust accurately. Material is ground off the blades by resharpening them, and adjustment is required when there is a need to change chip size, so the blades should be adjustable and firmly supported in the adjusted positions.
Reversible knives are often employed as a step in reducing machine down time and knife cost. A number of reversible-blade arrangements have been described, e.g., in Carpenter et al. U.S. Pat. No. 4,669,516 and in Svensson U.S. Pat. No. 4,047,670. Each of these has a knife blade of bent cross section. Carpenter et al. employs a V-shaped knife, and Svensson a Z-shaped knife. These each have angularly disposed cutting surfaces that make the blade extremely difficult to grip and support during assembly. The knives are clamped down by means of a clamp bar or hold down plate. The latter must be rather precisely machined to accept the complex shape of the knife blade. Due to the odd configuration, these blades are expensive to manufacture, and rather difficult to sharpen. The center waist or bend represents a weak sector, but is also the place of maximum mechanical stress. Thus, these blades are subject to an accelerated rate of failure under load. Consequently, both the initial cost of a chipper using these reversible knives and their cost of maintenance and operation remain relatively high.
In a disposable knife system, the conventional resharpenable chipper knife is replaced with an adapter or cassette and knife assembly. This can be comprised of a two-piece knife blade, a cassette, and a top hold-down plate. One such device is disclosed in U.S. Pat. No. 4,784,337. In this system a reversible, double-edge knife blade fits an elongated cassette that has a blade-receiving recess formed on it. The knife has a cutout on its underside which fits a rib or key on the cassette. The knife is held in place by fasteners, a retaining bar, or the like, so that it covers one edge, so that the other edge of the knife is accurately positioned at the entrance to a chip slot. Adjustment of blade position is effected by forming babbitt pads at the back or proximal end surface of the cassette.
The cutouts at the underside of the blades can have a weakening effect, unnecessarily leading to fracturing of the blades during a chipping operation. However, with the system of this patent, it is not possible to employ a keyless, flat blade.
Thus, present day log chippers can be provided with either large, single-edge knives or smaller, double-edge reversible knives. The large single-edge knives can be reground and replaced in the disk by babbitting, shimming, or otherwise adjusting the width dimension to be considered full width for use in the chipper disk clamping arrangement.
The smaller, double-edge blades now available require a unique profile which is critical to their clampability; this profile can be a key, groove, waist, etc. This structure can create a weak point that may fracture in service.
An ideal chipper knife should have a double-edge, reversible knife that can be discarded or resharpened, and which has flat upper and lower surfaces, i.e., without waists, bends, etc., so as to avoid fracturing. The knife should also be susceptible to dimensional adjustment, e.g., by babbitting or shimming, to place its non-exposed ground face into firm bearing contact with respect to a supporting land or surface in the associated cassette or holder. However, no such knife or knife assembly has been previously proposed.